In intermodal facilities, ports, railyards or other such facilities referred to herein as "shipping yards," containers are typically handled (i.e., lifted, lowered and transported) by a gantry crane having a wire rope hoisting system. Such a gantry crane usually has a rigid frame with vertical columns supporting two or more horizontal beams or tracks. An elevated hoisting system is mounted to the upper tracks. The hoisting system conventionally includes a trolley and a grappler which is movably suspended from the trolley for engaging, lifting, and lowering a standard container. The crane is equipped with wheels drivable by a conventional power source (e.g., hydraulic or electric motors) to enable movement of the crane around the shipping yard and to position the hoisting system over a container or stack of containers to be handled. Usually, the gantry crane also has a cab to occupy a human operator controlling the crane.
Conventionally, the grappler is suspended by wire ropes or cables. In particular, the grappler is conventionally suspended by one or more hoisting cable which is coilably paid out and/or retracted from a rotatable hoisting drum mounted on the overhead trolley. The grappler is lifted and lowered by selectively rotating the hoisting drum with a corresponding rotation.
The grappler and standard containers are cooperatively configured with standard dimensions. The grappler is conventionally rectangular, having four corner-mounted twistlocks configured and positioned to matably engage respective locking holes disposed in the top of a standard rectangular container. The twistlocks are remotely actuatable to be selectively locked with the locking holes, enabling the grappler to lift the container. Therefore, when a container is to be lifted by the crane, the operator must properly align the grappler relative to the container below so that the twistlocks are properly received in the respective locking holes on the container.
In shipping yards, containers must typically be loaded and/or unloaded from a standard chassis (e.g., a truck bed or a rail car). Typically, the gantry crane is driven over the container and stopped when the grappler is generally over the container. When positioned vertically over the container, the grappler is lowered by the hoisting cables so that the grappler twistlocks are received in the locking holes in the container. Thereafter, the grappler and container are elevated by the hoisting cables to lift the container from the chassis. The gantry crane can then carry and unload the container at a desired location (e.g., on the ground, on a pallet, on top of a stack of containers, on another chassis, etc.). The twistlocks are then disengaged from the container.
Because a grappler is suspended on flexible hoisting cables, the grappler is undesirably susceptible to swaying or pendulum movement. In particular, horizontal movement of the traveling crane is translated into pendulum movement of the grappler once the crane is stopped. The pendulum effect and the magnitude of grappler sway tend to increase with the paid-out length of the hoisting cables (i.e., the closer the grappler is to the ground). The swaying is most significant in a longitudinal direction corresponding to a forward-reverse axis along which the crane primarily travels.
The swaying of the grappler is problematic. Specifically, the swaying can frustrate the aligning of the grappler over a container to be lifted so that the twistlocks are received into the respective locking holes in the container. Also, swaying can add difficulty to accurately positioning a lifted load over a desired location for unloading. The crane operator must wait until the swinging of the grappler subsides. This results in undesirable waiting time to allow the swaying motion of the grappler to subside. Such waiting time directly effects the loading efficiency, loading turnaround time and profitability of a shipping yard.
It is desirable to dampen the sway of the suspended grappler. Dampening the sway reduces the amount of time needed for sway abatement. Thereby, the grappler is easier to align, and load handling times are desirably reduced, increasing loading efficiency.
Moreover, if the grappler is lowered or raised when the swaying has not yet abated, the grappler and wire rope system will be subject to increased load stresses as the grappler is lowered and raised compared to if it was not swaying. Such stress is undesirable and can potentially damage the grappler, the wire rope system, and any suspended load. Also, a swinging grappler presents a danger of inadvertently knocking the grappler into other objects. Thus, it is also desirable to dampen sway to minimize wear and tear on the components of the gantry crane.
A frequently-occurring grappler height requiring a substantial hoisting cable payout length is when the grappler is positioned to lift a container resting on a chassis. However, known sway-stabilizing systems have not been optimized for maximum anti-sway capabilities at a grappler height corresponding to one foot above the height of a standard shipping container on a standard chassis. Accordingly, known sway-stabilizing systems do not optimize shipping yard efficiency, because such systems are not designed maximizing sway dampening, and minimizing sway stabilization time, at the height that containers are most frequently lifted. Moreover, previous sway stabilizing systems have required complicated hydraulic systems to stabilize sway, disadvantageously increasing costs and the probability of mechanical failure.
An improved grappler sway-stabilizing system is needed which optimizes sway abatement and increases efficiency.